Science —

Is the Anthropocene a new geological epoch?

Researchers have suggested that the environmental changes triggered by …

These days, there seems to be a steady stream of species, from bats to bees, suffering from population collapses. In some cases, like fish, the cause is obvious; for others, like amphibians, the underlying cause (causes?) is less clear, but items such as environmental stresses and invasive species make for reasonable candidates. It's easy to wonder whether humanity's impact can really be so comprehensive, but there has been one sign that it is: geologists are seriously considering the possibility that we've triggered a new geologic epoch, which some are calling the Anthropocene.

I've seen the term appearing in a number of papers recently, and decided to look into it. It appears the term was only coined about six years ago, but it's gained serious credibility within scientific circles. Perhaps the most detailed treatment of the topic was published earlier this year in a member's journal of the Geological Society of America.

For the most part, geological periods are easy to recognize in retrospect. Major geological events are frequently accompanied by radical changes in the species present on earth; in some cases, the entire chemistry of the planet has changed. These sorts of radical changes leave a global mark on the planet, one that's easy to spot. The period we're now in, the Quaternary, is defined by regular glacial cycles, and our present interglacial warm period is called the Holocene. The paper considers the possibility that enough change is now apparent in the geologic record that we can consider the Holocene over with.

The case it builds is pretty compelling. Humanity is likely to have been causing extinctions since the Pleistocene, but the scale and scope during the Holocene is far larger. Most significantly, in the authors' view, is that it's extending into the oceans. Levels of atmospheric carbon dioxide are now a third higher than at any time in nearly a million years, and are likely to be double that by the end of the century. The temperatures at the high end of the range estimated by the IPCC haven't been seen in millions of years.

But the evidence goes well beyond these familiar signs. Sediment flow has radically accelerated as humans have cleared forests on all continents but Antarctica. The distribution of sediments has also been changed dramatically by the damming of most major river systems. Ocean acidification is likely to reach levels where calcium that's sedimented out of the oceans through biological activity will be leeched back out of those sediments. If the temperature estimates are right, global ocean levels are likely to rise by tens of meters due to a combination of thermal expansion of water and melting ice.

Any one of these events seems likely to produce a global signal that could easily be used to recognize the start of an era, the authors conclude. But they have a hard time deciding which, precisely, should actually be used to define the start of the Anthropocene in the same way that an iridium-rich layer defines the K/T boundary. After all, we've been causing extinctions for perhaps tens of thousands of years and land use changes have been going on since the start of agriculture. More pronounced signals, like the rise in atmospheric carbon, have occurred over hundreds of years, while its impact on things like ocean levels may take centuries to be apparent.

The authors also consider the use of isotopes from nuclear testing as a signature, but decide they appear a bit late in the game, after the Anthropocene is clearly underway. In the end, they decide not to make a decision on the matter.

The authors also decline to decide whether, in addition to ending the Holocene, human activity has brought the Quaternary period to a close. If carbon levels keep going up, they suggest we're likely to pop out of the relatively stable glacial cycling that has characterized the Quaternary. But, given the uncertainty in our expectations of human activity and climate feedbacks, they write "we prefer to remain conservative."